The invention relates generally to fiber optic communications and in particular to systems and methods for coupling optical fibers to integrated optical waveguides.
As fiber-optic communication advances to handling larger bandwidth, photonic integrated circuits or chips are expected to replace many of the discrete optical components that are currently used to construct optical communication systems. In systems combining optical fibers and photonic integrated circuits, light must be efficiently coupled between optical fibers and integrated optical channel waveguides. A practical coupling setup must be reliable, efficient, and capable of mass production at low per-unit cost.
Existing techniques for fixing an optical fiber (and lens) in position with respect to a channel waveguide include epoxy curing, soldering, mechanical fixture, and laser welding. In order to reduce the need for manual picking and placing/aligning of components in the packaging process, efforts have been focused on automating the process. For example, Newport, JDS-Uniphase and NEC are investing in the development of automatic parts-handling and assembly procedures using machine vision combined with micro-stages or micro-robots to achieve sub-micron precision [e.g., Soon Jang, “Automation manufacturing systems technology for opto-electronic device packaging,” 50th Electronic Components and Technology Conference, May 21-24, 2000, Las Vegas, Nev. USA; Peter Mueller and Bernd Valk, “Automated fiber attachment for 980 nm pump module,” 50th Electronic Components and Technology Conference, May 21-24, 2000, Las Vegas, Nev. USA; Kazuhiko Kurata, “Mass production techniques for optical modules,” 48th Electronic Components and Technology Conference, May 27-28, 1998, Seattle, Wash. USA].
In addition, silicon (Si) optical benches have become widely used. Such optical benches typically comprise a silicon substrate on which one or more grooves having a V-shaped or a U-shaped cross-section are wet-etched to guide the mounting or placement of photonic components including fibers, lenses and semiconductor chips [e.g.,Murphy, “Fiber-waveguide self alignment coupler,” U.S. Pat. No. 4,639,074, issued Jan. 27, 1987; Albares et al., “Optical fiber-to-channel waveguide coupler,” U.S. Pat. No. 4,930,854, issued Jun. 5, 1990; Benzoni et al., “Single in-line optical package,” U.S. Pat. No. 5,337,398, issued Aug. 9, 1994; Francis et al., “Waveguide coupler,” U.S. Pat. No. 5,552,092, issued Sep. 3, 1996; Harpin et al., “Assembly of an optical component and an optical waveguide,” U.S. Pat. No. 5,881,190, issued Mar. 9, 1999; Roff, “Package for an optoelectronic device,” U.S. Pat. No. 5,937,124, issued Aug. 10, 1999]. FIG. 1A shows an end view of a prior art V-groove alignment system for an optical fiber. Silicon substrate l40 has a V-groove 110 etched therein, in which optical fiber 120 rests. Channel waveguides may be mounted on substrate 140 using high precision automated alignment technology. Alternatively, as shown in FIG. 1B, a separate alignment step may be avoided by fabricating an integrated optical waveguide 130 and a properly aligned V-groove 110 on the same silicon substrate 140. Alignment between an optical fiber 120 and waveguide 130 is then achieved passively, by placing fiber 120 in V-groove 110.
Typically, V-groove 110 is formed by wet etching of substrate 140. As a consequence of the wet etching process, the end wall 150 of V-groove 110 is not vertical; it is inclined, as shown in FIG. 1B. When a cleaved optical fiber 120 with a substantially vertical flat end face 160 is placed in V-groove 110, end face 160 cannot make good contact with inclined end wall 150 of the V-groove and may not be able to make contact with the end face 170 of an integrated optical waveguide 130 fabricated or mounted flush with the top edge of end wall 150. If the distance between waveguide end face 170 and fiber end face 160 is large compared to the wavelength of transmitted light, then light is inefficiently coupled between the fiber and the waveguide. Thus, integrated optical waveguide 130 is typically positioned so that a substantial length of waveguide material protrudes into or overhangs V-groove 110, as indicated by dotted portion 137, thereby enabling end face 160 of fiber 120 to make contact with a protruded end face 131 of integrated optical waveguide 130.
However, in the structure of FIG. 1B, the protruding portion 137 of integrated optical channel waveguide 130 (and possibly its lower cladding layer 135, which may also act as a mechanical support) is vulnerable to damage when fiber 120 is placed in V-groove 110 and pushed into contact with waveguide end face 131. To protect waveguide 130, various mechanical stops as well as larger supporting and protecting structures have been used. These stops and structures may enhance the mechanical strength of the overhanging portion of the waveguide, prevent damaging contact with optical fibers, or provide a small gap between the waveguide and the fiber [e.g., Harpin et al., “Connection between an integrated optical waveguide and an optical fiber,” U.S. Pat. No. 5,787,214, issued Jul. 28, 1998]. In general, however, the addition of such stops and structures complicates the fabrication of the waveguide and the positioning and alignment of the optical fiber.
Another alternative is to create a vertical end wall for the V-groove using, for instance, a diamond saw-cut or ultrasonic grinding of the substrate [e.g., Bossler, “Method of attaching optical fibers to opto-electronic integrated circuits on silicon substrates,” U.S. Pat. No. 5,357,593, Oct. 18, 1994]. But because this approach is very time-consuming, it is not practical for applications involving a large number of V-grooves on a single wafer or mass production of V-grooved wafers.
An improved method of providing a connection between an optical fiber and an integrated optical waveguide is therefore needed.